1. Field of the Invention
This invention relates to thermo-optic light modulation and more particularly to a thermo-optic image bar for the modulation of light passing through a transparent medium in contact with an array of heating elements. Selective pulse activation of the heating elements produces temporary temperature gradients in the medium in the vicinity of the activated element, thus changing the refractive index of the medium and modulating the light.
2. Description of the Prior Art
U.S. Pat. No. 3,848,087 to R. M. Carrell discloses a scanning system which utilizes a multifaceted mirror for deflecting a light beam over a scanning area. As successive mirror facets rotate into the path of an incident light beam, the beam repetitively sweeps across the scanning area. Such an application has utility in both raster input scanning (RIS) and in raster output scanning (ROS). In RIS, the light beam illuminates an original document line by line and enables the information contained in that document to be encoded through procedures well known in the art. For ROS, the moving light beam may be used to encode information onto an information carrying media, such as a charged photoconductive member of an electrophotographic printer. A scanning system must encode information into the light beam and must distribute that information across the photoconductive member. To accomplish the encoding of the information, prior art ROS systems typically intensity modulate the light beam at controlled intervals. As the beam scans the photoconductive member, the modulation dictates which areas will remain charged and which areas will discharge.
Experience with prior art scanning systems has shown that the movement of the light beam across a facet of a rotating mirror reduces the effectiveness of the scanning system. It is more efficient, if the light beam follows or tracks a center of a particular facet as that facet moves in relation to the light beam. To provide this so called facet tracking, the light beam must be deflected in space before it contacts the spinning mirror facet. These two ROS requirements of intensity modulation and deflection for facet tracking are satisfied by well known acousto-optic modulator devices.
U.S. Pat. No. 4,047,795 to Hughes et al recognizes the possibility of deflecting an incoming laser beam using an optical integrated circuit which includes control electrodes for deflecting the beam in a controlled manner. Thus, the disadvantage of relatively costly acousto-optic modulating devices which are effective only over a limited optical wavelength has been overcome.
U.S. Pat. No. 4,396,246 to R. L. Holman discloses an electro-optic wave guide for both intensity modulation and continuous deflection of an incident laser beam, all on the same substrate. It has particular utility when used with a ROS to facet track and thus to deflect an information encoded laser beam to a charged photoconductive member. The optical coupling techniques needed are simplified and require no optical coupling between beam deflection and beam intensity modulation devices.
Article by S. G. Liu and W. L. Walters entitled "Optical Beam Deflection by Pulsed Temperature Gradients in Bulk GaAs," Proceeding of the IEEE (Correspondence), May 1965, pp. 522-523, discloses optical beam deflection in GaAs by establishing a transient temperature gradient in the material, which gives rise to a corresponding refractive-index gradient normal to the direction of the incident beam.
U.S. Pat. No. 3,623,795 to G. W. Taylor discloses a system which includes a material whose optical properties change sharply in a small temperature range. The material is heat biased to a temperature close to this range and a beam of light is directed at the material. In response to a signal applied to the material, its temperature is changed to reach the critical temperature where the optical properties of the material sharply changes a characteristic such as deflection angle, polarization direction or the like.
U.S. Pat. No. 4,376,568 to R. A. Sprague discloses a thick-film, electro-optic modulator. A laser beam is focused into the modulator, the beam expanding sideways so that a sheet of collimated light is provided. The collimated light is affected by an electric field from an array of electrodes on one side of the modulator and a broad electrode on the other side. The light diffracted by this electrode set is reimaged onto a recording medium with the zero order diffracted light blocked out. Thus, each electrode of the array on one side of the modulator acts as a light modulator for one picture element on the output.
U.S. Pat. No. 4,281,904 to R. A. Sprague et al discloses a total internal reflection type of electro-optic device which has an array of electrodes individually addressed. To record the displayed signal pattern, the electro-optic device is imaged as a line onto a recording plane, so that each individually addressed element of the electro-optic device acts as a light modulator or gate for one picture element along the recording line.
U.S. Pat. No. 3,499,112 to G. H. Heilmeier et al discloses an electro-optic display having a layer of nematic liquid crystal material and an array of electrodes. Light is scattered because of turbulence in the layer created by the application of a voltage across the layer in the regions between the electrodes having the voltage applied thereto.
The prior art light beam modulators are generally of the acousto-optic or electro-optic types. However, liquid crystal materials have also been considered as light modulators as seen above in reference to U.S. Pat. No. 3,499,112. Full width array image bars which act as light valves are also well known. They generally comprise a linear array of transparent electrodes which are positioned on both sides of a material such as temperature autostabilizing nonlinear dielectric elements (TANDEL) used in U.S. Pat. No. 3,623,795 or liquid crystal materials discussed in U.S. Pat. No. 4,386,836 to K. Aoki et al. Each electrode pair in the linear array momentarily passes or modulates a small beam of light whenever that electrode pair is electrically addressed, so that each burst of light impinging on a moving, precharged photoconductive member of an electrophotographic printer represents a picture element or pixel composing the background region of the latent image. Therefore, the latent image is produced line-by-line via the image bar through the erasure of background changes.
The publication entitled "Light-Switching Array for High-Resolution Pattern Generation" by B. Hill and K. P. Schmidt, pp. 169-174, July 1982, disclose a magneto-optic image bar called LiSA 512 Ehich is available from Amperex Electronic Corporation, Slaterville, R.I., an affiliate of North American Phillips. LiSA (Light Switching Array) is a chip consisting of a magneto-optic bismuth iron garnet film, grown epitaxially on a gadolinium gallium garnet substrate and etched into separate cells (refer to p. 170, FIG. 2). This film magnetizes spontaneously along an axis normal to the plane of the substrate. It exhibits the Faraday effect, so that plane polarized light passing through the cells are rotated, the sense of rotation depending on the direction of magnetization. In combination with an optical polarizer and analyzer, the cells effectively function as light valves controlled by the direction of magnetization (refer to FIG. 3). The LiSA chip uses a combination of heat and magnetic field pulses to switch the cells, thus passing short bursts of light in the typical image bar fashion. Since the film or chip is etched into relatively small cells, each cell will have uniform magnetization which resists changes in their magnetization, even against high magnetic fields for temperatures up to about 70.degree. C. By applying a heat pulse above 70.degree. C. to one or more of the cells followed with a magnetic field pulse, the cell magnetization can be switched from parallel to the direction normal to the chip substrate to non-parallel and vice versa. In FIG. 4 of this publication, a basic setup is shown, wherein a thin film resistive layer deposited on top of each light switching cell supplies the heat pulse and a separate winding around all the cells supplies the magnetic field pulse.
On page 171 of the publication, it is stated that the transmission coefficient varies with the wavelength of the transmitted light for both states of cell magnetization and that total blocking is only possible for monochromatic light of a specific wavelength. On page 172, it is stated that, in a practical printing system, the LiSA units would need a fiber optically coupled light source and a lens system to focus the light pattern onto a photoconductive member. Therefore, each cell on the chip would have to have its own individual fiber optic guide tube. Furthermore, all switching operations would be controlled by hybrid integrated circuits external to the LiSA chip so cooling would never be a problem.
The chief disadvantages of the prior art devices are that they are rather complex and expensive to manufacture. As will be seen below, a simple, cost effective full width array system or image bar which uses thermal energy pulses applied to a light passing medium in order to modulate light forms the basis of the present invention.